1. Field of Invention
Apparatuses and methods consistent with the present invention relates to a display and control thereof, and more particularly, to a display apparatus using a light emitting diode (LED) as a light source.
2. Description of the Related Art
Recently, an increasing number of display apparatuses use an LED or a laser diode (LD) as a light source. With the use of the LED or LD as the light source, a greater variety of colors can be expressed, compared to a conventional light source of lamp, and color and brightness can be easily controlled by adjusting current flowing in the LED or LD.
The LED includes a plurality of arrays for three colors of red, green, and blue, and is used as a backlight of a display apparatus.
A conventional driving apparatus driving such an LED can be formed as a circuit as shown in FIG. 1. In the driving apparatus of FIG. 1, a current setting unit 9 sets a target current value Iref to be supplied to the LED based on a brightness of an image, and a control block 11 applies a voltage to a gate terminal of a transistor 7 for the current supplied from a power source 1 to flow in the LED 3.
In this case, the LED 3 may be formed of one LED or a plurality of LEDs in series or in parallel. In addition, the transistor 7 may be provided as a metal oxide semiconductor field effect transistor (MOSFET), a bipolar junction transistor (BJT) or a semiconductor that performs a function of the MOSFET or the BJT.
When the current flows in the LED 3, a current detector 5 detects the level Io of the current flowing in the LED 3 and provides a detected current value Io to the control block 11. The control block 11 includes a digital to analog (D/A) converter and an operational amplifier, compares the target current value Iref with the current value Io detected by the current detector 5, and controls the amount of current flowing in the LED 3.
That is, the control block 11 converts a predetermined current value set as a digital signal from a current setting unit 9 into an analog signal, and compares the analog signal with the detected current value. In addition, the control block 11 adjusts a gate voltage applied to the gate terminal of the transistor 7 so as to make the detected current value equal to the target current value Iref.
According to the above method, the control block 11 can control a desired-current value Io to flow in the LED 3 when an input voltage Vin supplied from the power source 1 is kept higher than a forward voltage drop Vak by a predetermined value.
However, a reference voltage of the D/A converter of the control block 11 and an offset voltage of the operational amplifier may vary depending on the parts of the D/A converter or the operational amplifier. Accordingly, a substantial value of a current flowing in the LED 3 may be different even though the same target current value Iref is applied from the current setting unit 9. Particularly, it is difficult to control value of the current flowing in the LED 3 to be equal to the target current value Iref due to errors in the reference voltage of the D/A converter and the offset voltage of the operational amplifier.
For example, assume that the current setting unit 9 sets the target current value Iref to 0. However, the analog signal of the target current value Iref may be 0 or less than 0 or greater than 0 due to an unpredictable offset that can be generated when the target current Iref in the form of a digital signal is converted into the form of an analog signal. When the target current value Iref becomes greater than 0 and thus a little current flows in the LED 3, an image may be displayed on a screen with an unintended grayscale level.
In order to solve this problem, a circuit can be designed to apply a sufficient negative (−) offset to make the detected current value become 0. However, the control block 11 may apply a negative value to the gate terminal G of the transistor 7 if the detected current value becomes 0 by the negative offset. Since the transistor 7 is in a complete off state when the gate voltage has the negative value, a time delay occurs between a time at which the target current value Iref is set and a time at which a current value flowing in the LED 3 reaches the target current value Iref.